The invention relates generally to the fabrication of semiconductor devices and, more particularly, to the fabrication of a dielectric layer over a metal gate electrode in semiconductor devices.
As electronic devices become ever more complex, the need for greater and greater numbers of transistors on the device is increased. In addition, power consumption needs to be reduced while the speed of the devices needs to be increased. In responding to these problems, the area that each transistor occupies has been reduced considerably. However, this may adversely affect one or more of the other requirements. For example, as the transistor size is scaled down, the gate structure is also scaled down and this increases the resistance of the gate. Hence, power consumption is increased and the speed of the device may be decreased.
A number of efforts to reduce the sheet resistivity of the gate structures have been made in the past. For example, at one time the polysilicon was more heavily doped with either n-type or p-type dopants. At another time, a silicide was formed in the upper portion of the gate by reaction of the polysilicon with tungsten or titanium. Later, cobalt silicide was used to reduce the resistivity of the ever smaller geometries. At the present time, metal gate electrodes have been introduced and used in a number of applications.
Metal gate electrodes provide lower sheet resistivity virtually irrespective of the width of the gate. However, many metal gate materials have problems which must be overcome before they can be implemented in a standard semiconductor processing flow. One problem is that many metals are unstable next to silicon, which is commonly used to form the gate dielectric layer, i.e., as SiO2 or silicon oxynitride. Silicon oxynitride may be used for the dielectric spacer or liner formed over a metal gate electrode, due to characteristics such as forming a better barrier to migration of metal atoms, relative to SiO2.
Since many metals become less conductive in the form of the respective metal silicide (formed by reaction of the metal with silicon), the benefits of metal as the gate material may be compromised or lost if a metal silicide is formed as a result of formation of the silicon oxynitride spacer. Due to the ever-smaller dimensions of the metal gate electrode, if any portion of the metal gate electrode is converted into a less-conductive metal silicide, the effectiveness of the metal gate electrode as a conductor is reduced.
Thus, a need remains for a method of forming a silicon oxynitride dielectric layer over a metal gate electrode while forming no or substantially no silicide at the interface between the metal gate electrode and the silicon oxynitride dielectric layer.
The present invention relates to a process of manufacturing a semiconductor device including a metal gate electrode, comprising providing a semiconductor substrate; forming a metal gate electrode on the semiconductor substrate; forming by PECVD on a surface of the metal gate electrode a silicon oxynitride spacer, wherein the silicon oxynitride spacer is formed under initially silicon-starved conditions in which a quantity of at least one silicon-containing material is provided to a PECVD apparatus which is reduced relative to an amount of at least one other reactant, whereby substantially no silicide is formed at an interface of the metal gate electrode and the silicon oxynitride spacer.
In another embodiment, the present invention relates to a semiconductor device including a metal gate electrode, comprising a semiconductor substrate; a metal gate electrode; and a silicon oxynitride spacer formed on a surface of the metal gate electrode, wherein an interface of the first layer and the metal gate electrode is substantially free of metal silicide.
Thus, the present invention overcomes the problem of forming a dielectric layer on a metal gate electrode without forming a metal silicide.